In many applications where molding or spinning of precolored thermoplastic materials is used to prepare a colored article, it is desirable to have an evenly dispersed soluble dye in the resin as opposed to a pigment. Dyes commonly provide higher color strength, higher transparency and lower abrasion (in spinning of fibers) than pigments. A problem is, however, that no "true" black organic dye colorant exists.
Attempts have been made to produce adequate dark grays and black colors using organic dyes, usually by combining dyes of different colors, such as orange dyes and blue dyes. Unfortunately, these combinations require tedious process steps and produce products having inadequate color quantity. For instance, when a blue dye is mixed with an orange dye to form a black dye, the orange dye and the blue dye must fade at the same rate or the black will lose its color and exhibit an orange or blue tint. These and other problems have practically prevented the use of dyes as black colorants in thermoplastic materials.
Because no true black dye exists and attempts at combining dyes of differing colors have been unsuccessful, black pigments, such as carbon black, have traditionally been used to generate black coloration in thermoplastic materials. While carbon black produces materials having high black color strength, the use of carbon black in the processing of black thermoplastic materials results in problems with clogging of filter packs and erosion of spinnerets.
In addition, when carbon black is used to color fine denier thermoplastic fibers, such as fibers of less than about 4.0 denier/filament, weak fibers that are susceptible to breaking and fragmenting are formed. These weak fine denier fibers are a result of the size of the black pigment particles in relation to the fibers. For instance, as the fiber size decreases, the black pigment particles consume more and more of the diameter of the fiber, decreasing the amount of fiber-fiber bond at the location of the black particles. Hence, fine denier fibers containing pigment colorants generally result in fibers, which lack the strength for practical usage. Despite these drawbacks, carbon black has been the black colorant of choice in thermoplastic materials.
It is apparent from the above that there is a need for a black colorant that provides and maintains high black color quality in thermoplastic materials, such as molded articles, films and fibers. Furthermore, there is a need for a black dye composition that is small enough to function as a black colorant for fine denier fibers, such as fibers of less that 4 denier/filament.